1. Scope of the Invention
The present invention relates to Alxe2x80x94Mg based alloy sheets with good press formability, more specifically, excellent stretchability, superb deep drawability and high forming limits in the uniaxial tension to plane strain tension region. These Alxe2x80x94Mg based alloy sheets are suitable for automotive applications and the like.
2. Description of the Prior Art
From the consideration of recent concerns for the global environment, social demands toward reducing the weight of automobiles to reduce fuel consumption have escalated. To satisfy such demands, the application of aluminum materials to replace steel sheet for automotive parts has been investigated.
While aluminium alloy sheets have strengths at almost the same level as those of conventional steel sheets, they are generally poorer in press formability such as deep drawability and stretchability. As a result, the improvement of aluminium alloy sheets in terms of press formability has been strongly demanded by automotive manufacturers.
Conventionally, aluminum alloy materials of the Alxe2x80x94Mg series, such as JIS 5052 alloy or JIS 5182 alloy, and the Alxe2x80x94Mg based alloy material disclosed in Japanese Patent Laid-open No. Sho 52-141409, have been used for applications requiring excellent press formability. The present inventors have made investigations and conducted research, development, and merchandising, which led to the development of KS5030 alloy and KS5032 alloy (both under the trade names of Kobe Steel, Co.; the contents thereof are disclosed in Japanese Patent Laid-open Nos. Sho 60-125346, Sho 63-89649, Hei 2-269937 and Hei 3315486). These alloys are characterized as having high strength and high ductility due to the addition of a relatively high amount of Mg. These alloys have enhanced mechanical properties after painting and curing and better stress corrosion cracking resistance through the addition of Cu at about 0.5 wt. %. Additionally, these alloys have optimum grain sizes through the addition of Mn and Cr. These aluminium alloy sheets are used to make automotive parts and the like.
However, the formability of these aluminium based alloy sheets is not satisfactory for many applications, so automobile manufacturers have demanded further improvements in formability. One of the reasons why the formability is insufficient is because aluminium""s plastic anisotropy cannot be controlled well. No attention has been paid toward crystallographic texture control as a means to influence the plastic anisotropy of JIS alloys such as JIS 5182 or Alxe2x80x94Mg based alloys disclosed in Japanese Patent Laid-open Nos. Sho 52-141409, Sho 60-125346, Sho 63-89649, Hei 2-269937 and Hei 3-315486 wherein, only the chemical compositions of these alloys are specified. Hence, the formability is insufficient.
It has traditionally been known that crystallographic texture is an important nicrostructural feature in the control of the formability. It is known that the deep drawability of cold-rolled steel sheets can be improved by promoting a {111} texture, i.e., the normals of {111} crystallographic planes are nearly parallel to the sheet normal direction. It has been proposed in recent years that the formability of aluminium alloys can also be improved by controlling the crystallographic texture. For example, Japanese Patent Laid-open No. Hei 5-295476 discloses an Alxe2x80x94Mg based alloy sheet, wherein the volume fraction of the {110} texture (grains with {110} crystallographic planes nearly parallel to the sheet plane) is 10% or more, the ratio of the volume fraction of the {110} texture to the volume fraction of the {112} texture is 1.5 or more, and the grain size is the range of 35 to 80 xcexcm. However, the crystallographic texture disclosed therein is not optimum for deep drawing.
The Alxe2x80x94Mg alloy disclosed in Japanese Patent Laid-open No. Hei 8325663 was developed with attention focused on stretchability, while no consideration was paid to the grain structure which largely controls the drawability. Therefore, the alloy cannot achieve satisfactory press formability.
In an academic paper, by using computer simulations based on the theory of plastic deformation, P. Ratchev et al. made an assumption about the relationship between the crystallographic texture of Alxe2x80x94Mg alloy sheet and formability. He reported that a crystallographic texture with a strong Cube orientation might result in greater anisotropy, leading to the reduction of the formability (Texture and Microstructures, Vol.22, p.219, 1994).
It is the objective of the present invention to provide optimum Alxe2x80x94Mg based alloy sheets with excellent press formability by adjusting the volume fraction of various crystallographic texture components to control plastic anistropy, and by adjusting the type and amount of additional alloying elements to specific ranges in order to optimize grain size. Control of grain size and orientations should enhance the following three components of press formability:
1. stretchability;
2. deep drawability; and
3. forming limits between and including uniaxial tension and plane strain tension modes of deformation.
First, an Alxe2x80x94Mg based alloy sheet with good stretchability is generally achieved when its crystallographic texture is comprised of a volume fraction of grains around the CUBE* orientation in the range of 5 to 20%, a volume fraction of grains around the GOSS* orientation in the range of 1 to 5%, a volume fraction of grains around each of the BRASS*, S* and COPPER* orientations in the range of 1 to 10% and an average grain size in the range of about 20 to 70 xcexcm. Preferably, the texture is comprised of a volume fraction of grains around the CUBE orientation in the range of 5 to 15%, a volume fraction of grains around the GOSS orientation in the range of 1 to 3%, a volume fraction of grains around each of the BRASS, S and COPPER orientations in the range of 1 to 5%, and an average grain size in a range of 30 to 60 xcexcm.
*Defined in the Detailed Description of the Invention. 
Secondly, an Alxe2x80x94Mg based alloy sheet with good deep drawability is generally achieved when the ratio of the volume fraction of grains around the S orientation to the volume fraction of grains around the CUBE orientation (S/Cube) is 1 or more, when the volume fraction of grains around the GOSS orientation is 10% or less, and when the average grain size is in the range of about 20 to 100 xcexcm. Preferably, the ratio of the volume fraction of grains around the S orientation to the volume fraction of grains around the CUBE orientation (S/Cube) is 2 or more, the volume fraction of grains around the GOSS orientation is 5% or less and the average grain size is in the range of 40 to 80 xcexcm.
Finally, an Alxe2x80x94Mg based alloy sheet with higher forming limits in the region between uniaxial tension and plane strain tension region is generally achieved when the crystallographic texture is comprised of a volume fraction of grains around the CUBE orientation in the range of 30 to 50%, a volume fraction of grains around the BRASS orientation in the range of 10 to 20%, wherein the average grain size is in the range of 50 to 100 xcexcm. Preferably, the crystallographic texture is comprised of a volume fraction of grains around the CUBE orientation in the range of 40 to 50% and a volume fraction of grains around the BRASS orientation in the range of 15 to 20%, wherein the average grain size is in a range of 60 to 90 xcexcm.
Furthermore, all of these Alxe2x80x94Mg based alloy sheets have a composition preferably containing between 2 and 6 wt. % Mg and 0.03 wt. % or more in total of Fe, Mn, Cr, Zr, and/or Cu. (If Cu is added, it should be at 0.2 wt. % or more.) The balance of the composition is Al.
By appropriately controlling the crystallographic texture, grain size, and additional elements in Alxe2x80x94Mg based alloy sheets as described above, the press formability can be improved. More specifically, aluminium alloy sheets with excellent stretchability, deep drawability and/or with high forming limits in the region between uniaxial and plane strain tension can be achieved. These aluminium alloy sheets can be used preferably for automotive parts and the like.